As part of exchange mailbox migration I would be moving the mailbox from Exchange 2003 to 2010. Here I have question, Exchange 2003 mailboxes are already archival enabled , so when I move the mailbox to 2010 what happens to the archived data?
Will the user be able to access the archived data post mailbox migration?

Are there any best practice to follow in these situation where mailbox & archived data are getting moved to New Exchange 2010 & EV 9.0.1?

Your best bet is to upgrade the EV7 site to EV9 site and then start doing the migration via Move Archive, for the fact that EV7 and EV2007 don't have move archive.

Other than that you would have to do a PST Export of each mailbox and then import the PST in to the new archive.

So the way it really should be done is the following

1. Upgrade your EV7 site to EV2007 then to EV8 then to EV9 SP1
2. Apply the move archive hotfix in both EV9 SP1 environments
3. Disable Archiving for the mailboxes in the Old Site
4. Enable Archiving for the mailboxes in the new EV Site
5. All new items will be written to the new archive"
6. Users can get their old archive email by going to http://oldevsite/EnterpriseVault/ArchiveExplorerUI.aspx
7. Run the move archive wizard, moving items from the old archive to the new archive
8. After move archive has completed, you would then delete the old archives

The argument you may come up against is why don't you just upgrade the EV7 site to EV9 and keep everything the way it is, add the new exchange targets and then use synchinmigration mode, which is by far the simplest solution as for most cases, migrating to a new site is unnecessary.

Past that, if you can't upgrade you would have to go each archive in the Vault Admin Console, export the archives (split between retention categories) and then import those PST's in to each of the new sites archives.

The downside of this is, you lose things like the ArchivedDate that Move Archive would keep.

So for instance, say you have a Retention Category of Three years and you expire items by Archived Date.

If you archive an item on 03/01/2008, then the item will be deleted on or after 03/01/2011
If you use move archive, that item will still be deleted on or after 03/01/2011
However if you were to export the archive to a PST and reimport it in the new site today, the archived date would no longer be 03/01/2008, it would be 02/10/2011, meaning that it won't be deleted until 02/10/2014

In all honesty you may want to try engaging symantec professional services or third party consultancy like SterTech consulting or what not